Soft contact lenses offer a viable alternative to spectacles for the correction of visual defects such as myopia, hypermetropia and astigmatism. Early hydrogel lenses are derived almost exclusively from hydrophilic monomers such as 2-hydroxyethyl methacrylate, (HEMA). Although these lenses provided some comfort, they did not provide sufficient oxygen permeability to prevent problems associated with corneal hypoxia. Attempts to address this problem included co-polymerising HEMA with hydrophilic monomers such as methacrylic acid and N-vinyl pyrrolidone. Although these polymers increased the level of oxygen permeability, the incorporation of these co-monomers also leads to problems such as protein and lipid deposition, corneal desiccation, staining and lens dehydration.
More recently, a new generation of polymers has been developed to further increase the level of oxygen. These materials are based on the co-polymerisation of silicone methacrylates with hydrophilic co-monomers. The lenses produced from these materials were originally designed for extended wear, though daily wear products also exist now. Although successful in further increasing the oxygen permeability, these new materials still suffer from limitations such as lipid binding and dryness, all of which decrease lens on eye comfort.
There is therefore still a need for new contact lens polymers, which offer sufficient oxygen levels for normal corneal metabolism during daily wear and for extended wear, and which provide a high level of comfort throughout the day.
One class of polymers which holds considerable promise for novel contact lens materials are PEG based polyurethanes.
Urethane chemistries have also been widely investigated in the field of biomedical devices. For example, U.S. Pat. No. 3,786,034 discloses hard, hydrophilic polyurethane materials formed from reacting a specific polyol with a polyfunctional isocyanate. U.S. Pat. No. 3,821,486 teaches similar such materials. Likewise, U.S. Pat. No. 4,136,250 teaches a polymer formed by reacting a high molecular weight polydimethyl siloxane diol with 2 mole equivalents of isophorone di-isocyanate and then reacting with excess hydroxyl-containing monomers. Further urethane copolymers are disclosed in U.S. Pat. No. 4,454,309 and U.S. Pat. No. 4,359,553.
U.S. Pat. No. 6,930,196 discloses polyurethane hydrogel contact lenses prepared from prepolymers made by reacting (a) at least one multifunctional compound; (b) at least one di-isocyanate; and (c) at least one diol. The prepolymers so formed are then reacted with excess water to form a hydrogel polymer suitable for use as a contact lens.
U.S. Pat. No. 4,644,033 discloses a polyurethane hydrogel formed from the reaction of a polyoxyethylene and a polyfunctional isocyanate in a non-aqueous solvent. The materials can be molded into contact lenses.
U.S. Pat. No. 5,932,200 discloses polyurethane formed from reacting a diol component and an organic di-isocyanate with critical selection of the amount of water in the reaction mixture and the diol component. The polyurethane is in the form of a gel that has applications in burn/wound care dressings and as surgical implants.
U.S. Pat. No. 4,885,966 and U.S. Pat. No. 5,175,229 disclose hydrophilic polymeric soft contact lenses prepared from prepolymers that are isocyanate-capped oxyethylene-based diols or polyols having a molecular weight of about 7000 to 30,000, wherein essentially all of the OH groups are capped with isocyanate. The prepolymers are hydrated to form polyurea-polyurethane polymers that are characterised by having a non-ionic surface which is resistant to non-specific protein adsorption.
It is well known anecdotally that poly(ethylene glycol) based polyurethane copolymers are associated with poor storage stability. Known polyurethane polymers generally do not consistently maintain their properties after 6 months storage. The stability of polymers forming medical devices is clearly paramount. The properties of materials used in the manufacture of medical devices must be maintained upon extended periods of storage. The properties of such materials must be predictable and consistent. Materials having low and/or inconsistent storage stability are not suitable for use in the manufacture of medical devices such as contact lenses, regardless of any other promising properties.
The present invention seeks to provide new polyoxyethylene based polyurethane materials that are suitable for use in the contact lens industry. Ideally, the polyoxyethylene polyurethane-based materials of the invention exhibit exemplary physical properties, in particular, in terms of storage stability.